1. Field of the Invention
The invention is related generally to the field of roller cone drill bits. More particularly, the invention is related to methods for making the roller cones for a roller cone drill bit which provide the cones with increased resistance to stress-induced cracking.
2. Description of the Related Art
Roller cone drill bits known in the art include at least one, and generally three, roller cones rotatably mounted on corresponding legs which are joined to the bit body, the roller cones each typically have a plurality of cutting elements distributed about the exterior of the cone. The roller cone is typically made from steel or other high strength material. The cone may be treated in some areas on its surface to reduce susceptibility to wear. The cutting elements may be formed from the same base material as the cone, as is the case for so-called xe2x80x9cmilled toothxe2x80x9d bits, or may be separate elements inserted into sockets formed in the cone. This type of cutting element is referred to as an xe2x80x9cinsertxe2x80x9d. Inserts are generally made from tungsten carbide, diamond, boron nitride, or other individual or combinations of hard or superhard materials.
A typical prior art roller cone having insert type cutting elements is shown in FIG. 1. The cone is shown at 10, and the inserts are shown at 12. The inserts 12 are typically pressed into sockets (not shown in FIG. 1), where the external size of each insert and the internal size of the corresponding socket are such that the insert has an interference fit in the corresponding socket.
The stresses applied to the inserts and the cones during typical drilling operation are very high and are cyclical. The cyclical nature of the stress results from the fact that each cutting element is only in contact with rock being drilled for a portion of the time the drill bit is being rotated. As a result, it is common for the cone to crack within some of the sockets. Cracking in one or more of the sockets can result in loss of the inserts pressed into the sockets which undergo cracking, or can result in total cone failure.
Various methods have been developed to adjust the distribution of stresses in roller cones, with the objective of reducing cracking and insert loss. For example, U.S. Pat. No. 4,181,187 issued to Lumen describes cutting stress relief grooves in the bottom of the sockets. U.S. Pat. No. 3,970,158 issued to Black describes placing a compressible (malleable) material at the bottom of the sockets to absorb some of the cyclic stress applied to the inserts. The malleable material extrudes against the sides of the socket to reduce the incidence of cracking.
One aspect of the invention is a roller cone for a drill bit which includes a cone body having a plurality of sockets. An interior portion of at least one of the sockets is treated to provide residual compressive stress on an interior surface of the socket. In one embodiment, the treating includes shot peening. In another embodiment, the treating includes hammer peening. In another embodiment, the treating includes laser shock peening.
In one embodiment according to this aspect of the invention, the socket is treated over approximately the lower 50 percent of the lateral wall of the socket.
In another embodiment according to this aspect of the invention, the socket is treated on its bottom surface from the lateral wall inward approximately 33 percent of the diameter of the socket.
In a particular embodiment, the socket is treated over approximately the lower 50 percent of the lateral wall of the socket and the socket is treated on its bottom surface from the lateral wall inward approximately 33 percent of the diameter of the socket.
Another aspect of the invention is a roller cone for a drill bit including a cone body having a plurality of sockets formed in the cone. In this aspect of the, invention, an interior portion of at least one of the sockets is locally annealed on an interior surface of the socket.
In one embodiment, the socket is locally annealed over approximately the lower 50 percent of its lateral wall.
In another embodiment, the socket is locally annealed on the bottom surface from its lateral wall inward approximately 33 percent of the diameter of the socket.
In a particular embodiment, the socket is locally annealed over approximately the lower 50 percent of its lateral wall and the socket is locally annealed on the bottom surface from its lateral wall inward approximately 33 percent of the diameter of the socket.